My friend is always tired and he gets fatigue easily. can he be having chronic fatigue? will vitamin d supplement help his condition?

Should see a doctor: There are many possible causes of chronic fatigue- many type of infections (bacterial, viral, fungal, parasitic), anemia, nutrient deficiencies, toxic exposures, thyroid ; other hormonal problems, sleep disorders, heart disease, cancer, stress, depression etc.To effectively treat you need to determine the cause! see a good doctor, preferably one with a holistic perspective, for thorough evaluation.